The present invention relates to a wavelength-tunable semiconductor laser, particularly a wavelength-tunable semiconductor laser having an active part and an external waveguide part, the external waveguide part including a diffraction grating such as a distributed Bragg reflector (DBR). Such wavelength-tunable semiconductors have utility in known optical transmission systems, such as known coherent optical communication systems. Such a wavelength tunable semiconductor laser is used suitably as a laser for the local oscillation light source on the reception side of such coherent optical communication system.
A known wavelength-tunable semiconductor laser, having such active and external waveguide parts, is described in the article by Murata et al, "Spectral Characteristics of 1.5 .mu.m DBR DC-PBH Laser With Frequency Tuning Region", from IEEE Semiconductor Laser Conference B-3 (1983). This article discloses frequency tunable (wavelength-tunable) single longitudinal mode semiconductor lasers, wherein tuning is performed by injecting current into a distributed Bragg reflector (DBR) region of the external waveguide part, the current injection reducing the refractive index of the DBR and changing the Bragg wavelength. This article discloses that wide continuous frequency tuning with stable spectral linewidth oscillation can be obtained.
The specific structure of the semiconductor laser disclosed in Murata et al includes two current injection regions, an active region and a DBR (external waveguide) region, the composition and thickness of the waveguide layers being the same for both regions. This article further discloses that between the active and waveguide layers is formed an InP buffer layer in order to remove the active layer in the DBR selectively by preferential etchant. This article discloses that the active and DBR regions were isolated electrically by a 20 .mu.m wide etched groove which was formed on both sides of the center stripe area of the device.
This conventional semiconductor laser, described in the Murata et al. article, utilizes injection of a current to the external waveguide (DBR) region to change a carrier density of the waveguide, so as to change the refractive index of the external waveguide and thereby change the Bragg reflection wavelength of the diffraction grating to make the oscillation wavelength variable.
While this article discloses a wavelength-tunable semiconductor laser, some problems still remain in the operation of this device. Since current injection is used to vary the refractive index of the external waveguide to make the oscillation wavelength variable, this conventional structure can only modify the refractive index, and oscillation wavelength, a relatively small amount. For example, the change of the refractive index relative to the current injection quantity is at most about 3.times.10.sup.-3, and the wavelength change (at a wavelength of 1.5 .mu.m) by current injection is as small as about 10 .ANG..
Moreover, since this conventional structure uses current injection quantity (or, in other words, the change in carrier density) to change the refractive index of the external waveguide and vary the oscillation wavelength, the switching time of the device (that is, the time to change the refractive index), which is determined by the lifetime of the carriers, is as low as a few (e.g., three) nsec.
Thus, while the semiconductor laser disclosed in the article by Murata et al, can be utilized to vary the wwavelength of light emitted from the active layer of the semiconductor laser, problems still remain, with respect to increasing the range of variation of wavelength and improving switching time for varying the wavelength.